Ensure schema inspection always runs once even if the builder is used again

[rstoyanchev]

Currently DefaultSchemaResourceGraphQlSourceBuilder registers a Consumer<GraphQL.Builder> callback to run inspection at the end of the build steps. This is because SchemaMappingInspector depends on the following:

1. Code registrations in the RuntimeWiring, which is available only in DefaultSchemaResourceGraphQlSourceBuilder#initGraphQlSchema.
2. Access to GraphQLSchema after it is transformed with type visitors which happens later in the base AbstractGraphQlSource.

The approach works fine if the builder is used once, but has the side effect of registering an additional callback with each call to build(), which hasn't been noticed as the builder is normally used once. However, GraphQlSource was always meant to allow resolving the GraphQL and GraphQLSchema at runtime, and we should remove this side effect.